Sebastian's Sister

One morning, Sebastian receives a letter with no return address. He does not let his young master see it despite his orders. Three days later, a mysterious girl arrives in Lady Elizabeth's carriage. She seems obnoxious at first, but as she stays longer, Ciel wants to find out more about this girl who has a lking towards Sebastian. Ciel gets jealous of her and Sebastian, even though he is engaged. Slowly, but surely, the 18 year old earl falls in love with this mysterious but lively girl who seems timeless.
Disclaimer: Black Butler owns all the characters except the ones I have made up.